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SENSORY SYSTEM LECTURE 1 RECEPTORS

SENSORY SYSTEM LECTURE 1 RECEPTORS. DR. ZAHOOR ALI SHAIKH. Sensory system or Input system. What type of sensation do you know? Touch Fine Crude Pressure Position Vibration Two point discrimination Pain Temperature Cold Warm Stereognosis. How we feel the sensation?.

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SENSORY SYSTEM LECTURE 1 RECEPTORS

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  1. SENSORY SYSTEMLECTURE 1RECEPTORS DR. ZAHOOR ALI SHAIKH

  2. Sensory system or Input system • What type of sensation do you know? • Touch • Fine • Crude • Pressure • Position • Vibration • Two point discrimination • Pain • Temperature • Cold • Warm • Stereognosis

  3. How we feel the sensation? • Sensory receptor • Sensory pathway • Sensory cortex

  4. Cont….COMPONENTS • Receptors • Peripheral nerves • Spinal cord • Tracts • Thalamus • Thalamocortical projection • Somatosensory cortex

  5. 2nd order Thalamus 3rd order Sensory Cortex SI & SII Tracts Medullary Nuclei (Localization & Perception of sensation) Dorsal Horn Of Spinal cord 1st order Peripheral Nerve Receptor Stimulus

  6. Sensory Receptors • These are transducers that convert various forms of energy into action potentials in neurons. • The particular form of energy to which a receptor is most sensitive is called as its adequate stimulus.

  7. Adequate stimulus Each type of receptor is most sensitive to a specific form of energy, called adequate stimulus, the receptor is almost non-responsive to the normal intensities of other forms of energy. e.g. rods & cones are stimulated by light not heat

  8. Types of receptors

  9. Free nerve ending Epithelium of Skin Non adapting Pain

  10. Schematic representation of sensory nerve endings in the skin: • Free nerve endings in the dermis and epidermis (Pain) • Tactile corpuscle (Meissner) in a dermal papilla (Touch) • Bulbous corpuscle (Krause) in the dermis (Cold) • End organ (Ruffini) in the dermis (Heat) • Nerve terminals around hair follicles (Touch) • Free nerve terminals in a hair papilla and deep layers of dermis (Pain) • Lamellated corpuscle (Vater-Pacini) in superficial fascia (Pressure)

  11. Receptor Properties Receptor potential or Generator Potential Transduction (excitability) • When we apply pressure, we generate depolarization in the receptor it is called Generator potential or Receptor potential. • Non propagated depolarizing potential.

  12. Transduction (receptor potential) • It is a change in membrane potential of a receptor. • Due to: 1) Mechanical deformation of receptor membrane. 2) Application of chemical which binds to the receptor. 3) Change in temperature. 4) Application of electromagnetic energy. • These lead to the opening of selective channels and depolarization of receptor membrane.

  13. Relation of Receptor potential to Action potential: When we increase the pressure, magnitude of receptor potential rises, when it reaches to threshold level, action potential is generated in the sensory fiber attached to the receptor Let us take the example of Pacinian Corpuscle . Corpuscle has central nerve extending through its core.When pressure stimulus is applied  the covering of the corpuscle and central nerve fiber will be deformed  opening of Na+ channels  Na+ influx (to the interior of the nerve fiber)  creates high positivity inside the fiber  generator potential  then the generator potential depolarizes the sensory nerve at the 1st node of Ranvier. Once the firing level is reached, action potential is produced in the nerve fiber.

  14. The frequency of AP is proportionate to the intensity (magnitude) of the applied stimuli • In other way: the more the receptor potential rises above threshold level, the greater becomes the action potential frequency What is the difference between generator potential and action potential? • Action potential • In the Sensory Nerve fiber • Not Graded • Obeys all or non rule • Not summated • Propagated • Receptor or Generator potential • In the Receptor • Graded • Doesn’t obey all or none rule • Can be summated • Unpropagated

  15. RECEPTOR POTENTIAL & ACTION POTENTIAL

  16. Note: if stimulus to the receptor is sub threshold then no action potential is generated in sensory nerve fiber Relation between receptor potential and action potential of sensory nerve

  17. Sensory Transduction

  18. Adaptation or Desensitization • When stimulus of constant strength is applied to a receptor, the frequency of action potential in its sensory nerve decreases over the time. This is called adaptation. Degree of adaptation varies in different receptors.

  19. Classification of receptors • Rapidly adapting or phasic receptors e.g. pacinian corpuscles. • Slowly or non adapting or tonic receptors e.g. Nociceptors, muscle spindles.

  20. ADAPTATION OF RECEPTOR

  21. RESPONSE OF PHASIC & TONIC MECHANORECEPTOR

  22. Why we feel touch, pain, warm sensation when all sensory nerve carry the information to brain as action potential? Coding of sensory information

  23. Coding of sensory information • Due to the Doctrine of specific nerve energies. We have; • Specific receptors. • Specific sensory pathways. • Specific part of the brain they activate.

  24. Law of projection • If we stimulate sensory pathway along its course to the sensory cortex, the conscious sensation produced is refered to the location of receptors. This principle is called as Law of projection. • If we stimulate sensory cortex area which receives impulses from left hand, patient reports sensation in the left hand, not in the head.

  25. Cont….. Q. In patients whose limb was amputed (cut off) they complained of pain in the absent limb (Phantom limb), Why? Due to law of projection, the ends of nerves cut at the time of amputation when stimulated, sensation evoked are projected to where the receptor used to be present,i.e. in the limb.

  26. Sensory Unit • Single sensory axon with all its peripheral branches. Receptor field • Receptor field of a sensory unit is the area from which a stimulus produces response in that unit.

  27. RECEPTIVE FIELD

  28. Recruitment of sensory units • Weak stimulus activates receptors with lowest threshold. • Strong stimulus activates those receptors also which have high threshold .

  29. Intensity discrimination Spatial summation • Stimulus intensity which causes increase firing of many units (fibers ) due to overlap of one sensory unit to the other.

  30. SPATIAL SUMMATION

  31. Temporal summation • If we give stimulus which causes increase frequency of nerve impulses in single sensory fiber. (same sensory unit)

  32. Physiological classification of nerve fibers that transmit different types of sensations

  33. Physiological classification of nerve fibers that transmit different types of sensations Number Origin Fiber type Ia Ib II III IV Muscle spindle, annulo-spiral ending. Golgi tendon organ. Muscle spindle, flower-spray ending, touch, pressure Pain and cold receptors; some touch receptors Pain, temperature, and other receptors A A A A Dorsal root C

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